Abstract: The disclosure provides a fluorescent sensor, comprising a) a responsive polypeptide, and b) an optically active polypeptide, wherein the optically active polypeptide is inserted into the responsive polypeptide. The present disclosure also relates to a nucleic acid sequence encoding the fluorescent sensor according to any embodiment, or a complementary sequence thereof. The disclosure also provides an expression vector comprising the nucleic acid sequence or a complementary sequence thereof of the disclosure operably linked to an expression control sequence. The disclosure also provides a cell containing the expression vector of the disclosure. The present disclosure also provides a method for preparing the fluorescent sensor of this disclosure, comprising the following steps: providing a host cell comprising an expression vector that expresses the fluorescent sensor of the disclosure, culturing the host cell under conditions suitable for the expression of the host cell, and separating the fluorescent sensor.